The field of the present invention is screens employed for separation of product using vibrational energy.
Vibratory screen systems have long been employed in both circular and rectangular form. The devices typically include a resiliently mounted housing having a screen extended across the housing. A vibration generating drive is coupled with the housing to vibrate the screen in an advantageous manner to increase screening efficiency. The screens are either self contained by including screen cloth tensioned and bonded to a frame or rely on mechanisms on the resiliently mounted housing for placement and tensioning: In the latter circumstance, the screen typically includes screen cloth to which may be mounted hooks or eyes for attachment of tensioning mechanisms associated with the housing.
Alternatively, screens can include a perforated plate with screen cloth bonded thereto. When a plate is used, the screen may be tensioned before bonding to the plate. The screen cloth may be bonded to the plate by a layer of epoxy or thermoplastic material. The bonding material is positioned on the plate and the screen tensioned thereover. The material is then treated, commonly by heating to either initiate curing of the epoxy or fusing of the thermoplastic material. Nonstick layers of PTFE sheet may be employed where the assembly is compressed during the curing or fusing step. Multiple layers of screen cloth are known to be used in such assemblies. The plates include interstices for the passage of the screened material therethrough.
Screens which employ hooks or eyes for tensioning by a separate mechanism having laminated layers have also been known. Bonding to frames by spot welding, epoxy or fusible material are known. Further, fusing multiple layers of screen cloth into the top of a frame structure made of fusible material having a peripheral frame and a pattern of open cells defined by cell walls has been previously known. The multiple screen cloths are bonded to the frame and the cell walls by fusing the frame structure and resolidifying it after impregnation through the screen cloth or cloths. Such a structure is disclosed in U.S. Pat. No. 5,851,393, the disclosure of which is incorporated herein by reference. Backup layers have been coated with epoxy and bonded to filter cloth such as disclosed in U.S. Pat. No. 5,814,218. Diffusion bonding is practiced between metal screens. The layers of screen cloth are pressed together and subjected to substantial heat for an extended time. No bonding material is used in the diffusion bonding process.
A laminated screen having two or more woven screen cloths is also known which has threads in one of the cloths having surface portions which are fusible below a temperature at which the other woven screen cloth is heat affected. Heat effects to be avoided are changes in the physical and chemical properties of the screen cloth. These threads with surface portions fusible below a temperature at which the other woven screen cloth is heat affected are woven into the cloth. The screen cloths are of different mesh size with the courser mesh including the threads with fusible surface material. The fusible surface material is fused into the other screen cloth at the knuckle contacts of these threads with the finer screen cloth. Threads with fusible surface portions may be dispersed within the screen cloth to best advantage. Such threads may be arranged in only one direction of the screen cloth. Such threads may be spaced apart with conventional threads therebetween. The threads with fusible surface portions may additionally be fusible fully therethrough. The screen cloth threads may be metal wire such as stainless steel. This system is disclosed in U.S. Pat. No. 6,431,368, the disclosure of which is incorporated herein by reference.
The foregoing vibratory screen has been found to provide substantial advantage in many industrial uses. In such uses, the lower supporting layer commonly employs a 20 mesh wire cloth. This mesh size provides sufficient support for the finer screens mounted above and affixed to the 20 mesh screen as described in U.S. Pat. No. 6,431,368. The assembly provides a degree of flexibility which enhances screening efficiency. However, such screening structures lack sufficient tensioning capability and underlying structural support to fully satisfy some applications such as cleaning recirculating drilling mud. However, to conventionally bond a plate or more rigid screen able to withstand such tensioning reduces the screening efficiency of the finer mesh.